The present invention relates to an illumination apparatus for illuminating an original document in an electrostatic copying machine or the like.
Continuous efforts have been made in the art of electrostatic copying machine to increase the speed of operation. In one type of copying machine an original document is placed face down on a transparent platen and illuminated from below. An illumination apparatus generally comprising a lamp and a curved reflector is moved together with an optical system to scan the document. A light image of the document is focussed by the optical system through a slit onto a moving photoconductive member such as a drum to form an electrostatic image through localized photoconduction. The electrostatic image is developed to form a toner image which is transferred and fixed to a copy sheet to provide a hard copy of the original document.
In this type of copying machine both the optical system and exposure apparatus must be reciprocated once to make each copy. The optical system may be designed so that it is necessary to reciprocate only one or two small plane mirrors, thereby constituting no significant problem. However, the exposure apparatus is much bulkier than the mirrors, and tends to vibrate when moved at high copying speeds. This causes the mirrors and other elements of the optical system to also vibrate and degrade the image quality. In addition, the life of the lamp is reduced by the shocks caused by rapid reversal of direction of movement. It is not desirable to rotatingly reciprocate the drum due to the weight thereof, but to rotate the drum at constant speed in one direction.
In such a copying machine it is also desirable to illuminate the edges of the document with greater intensity than the center thereof. This is since an uncompensated light image will be darker at the edges than at the center in accordance with the well known cos.sup.4 .theta. rule. The increased illumination at the edges cancels out the cos.sup.4 .theta. effect and produces a uniform light image. Such compensation has been difficult and complicated to accomplish in prior art copying machines with the type of illumination apparatus in which the illumination lamp and reflector are moved.
Another type of copying machine comprises a photoconductive member having a flat surface such as a sheet or endless belt. The illumination apparatus and optical system are fixed. The illumination system illuminates the entire document with a high intensity flash and the optical system focuses a light image of the entire document onto the photoconductive member. Whereas this type of copying machine eliminates the problems encountered in moving illumination and optical components at high speeds for scanning, it increases the overall size and weight of the copying machine. The electrical power required to produce the high intensity flash is much greater than that required in a scanning type illumination apparatus. As yet another disadvantage in a flash exposure copying machine, the photoconductive member must be in planar form, such as a sheet or belt. Such photoconductive members are much less durable than a cylindrical photoconductive drum and require repair and replacement at frequent intervals.